1. Field of the Invention
The present invention relates to a developer such as a monocomponent developer, nonmagnetic monocomponent developer, two-component developer and the like for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing and the like.
2. Description of the Related Art
In recent years demand has developed for toners used for image formation which have excellent low-temperature fixing characteristics so as to conserve energy consumption in image forming apparatus. Realizing excellent low-temperature fixing characteristics not only conserves energy consumption by the image forming apparatus, but also reduces the warmup time required from the moment the power source is switched on until developing is possible, thereby providing superior operationality.
Demand has also developed for more compact image forming apparatus. More compact fixing rollers are required in conjunction with compact image forming apparatus. Thus, smaller diameter fixing rollers must be used with the compact image forming apparatus. When the diameter of the fixing roller is reduced, the nip width between pairs fixing rollers is also reduced, thereby creating demand for toner having better fixing characteristics. This demand is particularly acute in the case of low-temperature fixing.
On the other hand, when the fixing roller is made more compactly, the heat capacity of the roller is reduced, such that when power is supplied to the built-in heater of the fixing roller when the image forming apparatus is started, the surface temperature of the roller temporarily attains a high temperature of 200.degree. C. or greater. Therefore, at initial start up of the image forming apparatus, the toner must have high-temperature offset characteristics until the fixing roller temperature reaches a normal temperature which is lower than the above-mentioned peak temperature.
Heretofore, toner using offset preventing agents have been variously proposed in order to obtain a toner which is fixable within a wide temperature range from low temperature to high temperature in accordance with the demand described above. However, since offset preventing agents are the cause of frequent filming on the developing sleeve and photosensitive member, as well as toner fogging, toner having enough performance has not yet to be produced.
For example, Japanese Unexamined Patent Application No. HEI 03-5764 disclosed a toner using a low-melting point carnauba wax having an acid group as a release agent. However, a problem of offset occurs in the high temperature range although suitable offset preventing characteristics are obtained in the low temperature range.
Japanese Unexamined Patent Application No. SHO 60-252366 discloses a toner containing two types of release agents and a binder resin, and proposes combining a nonpolar wax and a wax having an acid group. This invention, however, has disadvantages insofar as in methods which develop by regulating the amount of toner on the surface of a toner-bearing member which are typical of nonmagnetic monocomponent developing methods, free wax causes filming of the toner-bearing member, thereby producing image noise such as reduced image density and the like.
Therefore, it is difficult to eliminate the aforesaid disadvantages by simply using two types of offset preventing agent as described above.
When viewed from the perspective of a compact form-factor of the image forming apparatus, however, monocomponent developing methods which do not use a carrier is beneficial. Monocomponent developers do not require mixing mechanisms because they do not use carriers, and such developers can produce stable images by means of simple and compact developing devices because carrier replacement is unnecessary. Until now, however, a monocomponent developer having adequate fixing characteristics has not been available.